Phase change behavior of ammonia-water mixtures in corrugated plate heat exchangers

The present report describes the work realized in the frame of a Mas- ter Pro ject in collaboration between the company EnBW Energie Baden- Württemberg AG (Germany) and the Section of Mechanical Engineering of the Swiss Institute of Technology of Lausanne (EPFL) in view to obtain 30 ECTS credits and the title of Master of Science in Mechanical Engineering. The general goal is to reach a better understanding of the heat exchange processes occuring in corrugated plate heat exchangers. EnBW AG operates a geothermal power plant based on the Kalina cycle, applying corrugated plate heat exchangers. As working fluid of the power cycle a zeotropic mixture formed by ammonia and water is used. Heat source of the geothermal power plant in Bruchsal (D) is a natural hydrothermal system. Zeotropic mixtures present specific properties complicating their use in heat exchangers. The mix- ture components' different boiling points create a temperature glide during the mixture's phase change, in contrary to pure fluids that vaporize at a constant temperature. In addition, a composition difference appears between the liquid phase and the vapor phase, which induces a diffusion mass transfer resistance. The several studies conducted on the topic show that both effects combined reduce the heat transfer performances in phase change. The modelling of the mixture properties and behavior is realized in an in- dependent manner by using correlations from the literature. Specific enthalpy and entropy are notably modelled with the correlations by Ziegler and Trepp. The heat transfer coefficients are computed using correlations by Dovíc et al. for the monophasic flows and by Han et al. for biphasic flows. The calcula- tion of pressure drops is discussed and correlations by the same authors are presented. Unfortunately the pressure drop calculation could not be imple- mented in the time allowed to this work. However their implementation does not present special technical difficulties. The results of the calculation for realistic conditions show good agreement with the available experimental data. More data are however needed to obtain a complete validation of the model. A sensitivity analysis is performed to determine the effect of the change of certain operating conditions or of the heat exchanger design. The influence of the following parameters is studied: number of plates, length of those, geothermal brine inlet temperature and mass flow and ammonia concentration of the mixture. It appears that the most influent parameter on the recovered heat load is the inlet brine temperature. Adding plates to the heat exchanger only permits a limited gain of performances. Lengthening the plates is pre- ferred. An augmentation of the mass velocity of the brine, which seems to be the limiting factor in the heat transfer coefficients values, seems also not to permit a significant performance increase. The sensitivity analysis shows though that it is possible to react to a brine temperature change by modifying the ammonia concentration of the mixture. It is therefore possible to adapt the cycle to the conditions, as one wants to maintain a constant turbine inlet temperature or to reach better exergy efficiency. This result must however be reconsidered through a wider perspective including the entire thermodynamic cycle. Given the results listed here above, one can deduce that the relatively low temperature of the geothermal brine strongly limits the possibilities of 3 optimization. The adapted ammonia concentration range is narrow. In certain cases the ob jective of maintaining the amount of transferred thermal power and the one of keeping good exergy efficiency are conflicting. It could however be instructive to lead a sensitivity analysis on the cycle pressure level. This analysis would though have to be included in a global view of the cycle given the numerous consequences. Finally, the results produced by the model allow to clearly illustrating the advantage of zeotropic mixtures regarding the exergy efficiency of the heat exchange. The efficiency is very high, reaching 90% in certain conditions, which is not realizable in the same conditions using a pure working fluid. The study of the behavior of zeotropic fluids should be pursued in near future in the view of a more rational use of the energy.